A prior art device of this type is described in British published patent application No. GB 2 000 632.
The simplest embodiments of devices for detecting neutral particles comprise an anode wire and a cathode which acts as a converter, with the anode wire and the cathode being disposed in the vicinity of each other, and with the assembly being contained in a gas-filled enclosure.
An example of a detector having this structure is described in the article at page 577 of "Review of Scientific Instruments", vol. 50, No. 5 (1979).
Regardless of the way in which such detectors operate, the function of the converter included therein is to emit ionizing particles under the impact of neutral particles, and more specifically to emit fast or "primary" electrons. The kinetic energy of such fast electrons is greater than the ionization energy of the atoms or molecules of the gas and is sometimes so high that these electrons cannot be effectively guided towards the anode by the electric field established inside the enclosure.
In contrast, by ionizing the gas these primary electrons create electron/positive ion pairs, and the electrons created in this way are referred to as "secondary electrons". The secondary electrons migrate towards the anode while the positive ions migrate towards the cathode.
An electric charge whose magnitude depends on the number of secondary electrons is thus collected at the anode. An appropriate circuit (or "channel") connected to the anode delivers a measurement signal proportional to the electric charge collected by the anode, and consequently representative of the energy or the intensity of the observed phenomenon.
In conventional manner, the collected charge may be detected either directly or indirectly. When detected indirectly, the electric charge is allowed to flow along a conductor and detection consists in detecting the resulting electric current.
Apart from these common characteristics, neutral particle detectors are very diverse, and are generally classified into two main types, depending on whether or not they make use of the "Townsend avalanche" phenomenon.
This phenomenon, which is well known to the person skilled in the art, appears when the electric field in the vicinity of the anode is sufficiently intense to accelerate the secondary electrons so that the kinetic energy which they acquire between two collisions becomes greater than the ionization energy of the atoms or the molecules of the gas.
Under these conditions, the secondary electrons may in turn create further secondary electrons which themselves participate in an avalanche phenomenon as they move towards the anode, with the avalanche ceasing only when all of the free electrons have been collected by the anode.
This multiplicative effect on the detected electrons gives rise to significant amplification of the measurement signal, e.g. by as much as 10.sup.7, thereby very substantially improving the signal/noise ratio.
Under good conditions, the number of secondary electrons can be maintained at a high value determined ratio to the number of primary electrons. Detectors operating under such conditions are called "proportional counters".
However, the multiplication factor of the avalanche phenomenon can only be controlled to a limited degree of accuracy, and in some applications it is preferable to provide detectors which do not make use of the avalanche phenomenon. This is true, in particular, of "ionization chamber" detectors and examples thereof are given in the above-mentioned article in "Review of Scientific Instruments", in British Pat. No. GB 1 578 325, and in French Pat. No. FR 2 503 381. These instruments generally detect radiation under metrological conditions so long as the charge accumulated on the anode is measurable, which requires the neutral particle radiation to be intense or the duration of measurement to be long.
That is why ionization chambers cannot be used in imaging, or in localizing particles, except under very special radiation conditions. In particular, they cannot be used except if the energy of the radiation to be detected is about 80 keV or rather less than said value. Further, the application of ionization chambers to imaging gives rise to complex devices, since each chamber provides only one point (or "pixel") of the image, as shown in French Pat. No. FR 2 503 381.
Avalanche detectors which are substantially better adapted than ionization chambers to such applications are well known to the person skilled in the art of imaging or in the art of localizing particles.
An example of a proportional counter applied to two-dimensional image acquisition is given in the article on page 157 of "IEEE Transactions on Nuclear Science", vol. 27, No. 1 (February 1980).
This detector comprises a two-dimensional network of anode wires associated with bundles of lead oxide crystal tubes acting as converters and disposed perpendicularly to the plane of the anode network. The angle of incidence of the neutral praticle radiation on the detector is close to 90.degree. and the conversion efficiency of the detector is fairly independent of the angle of incidence and remains less than 8%.
Another proportional counter for direct acquisition of two-dimensional images is described in British Pat. No. 2 000 632. This device, like the above device, belongs to the class of apparatuses known as multi-wire proportional chambers and is intended for detecting radiation impinging thereon at an incidence perpendicular to its plane. It comprises two cathodes in the form of strips and constituting solid converters together with an anode constituted by a plane of wires which are electrically interconnected.
The strips of cathodes are associated with delay lines which are in turn connected to a time analysis circuit.
Other than the fact that this detector requires a complex analysis circuit, and the fact that its conversion efficiency cannot be very high, it necessarily leads to rather long image acquisition times, thereby limiting its application to the formation of quasi-static images, for example to scintigraphy.
In this context, the essential aim of the invention is to provide a device for detecting and localizing neutral particles, said device being substantially more efficient than prior multi-wire proportional chambers, in particularly when the energy of the neutral particles is substantially greater than 50 keV.